| Summary: | We report on the observation of vortex instabilities in the wake of a tidal turbine undergoing axially
oscillating inflow, using unsteady RANS simulations. Unsteady inflow causes the turbine blades to shed vorticity of
time-varying strength into the wake, which in turn leads
to a spatial variation in wake vortex strength, such that the
vorticity of adjacent returning wake segments can differ.
This spatial variation triggers the instability of tip vortices,
the characteristics of which are shown to be governed by
the ’frequency ratio’ ω/ΩNB (where ω is harmonic inflow
frequency, Ω is turbine rotational frequency and NB is
the number of blades). If ω/ΩNB = m, where m is an
integer, the wake is stable. If ω/ΩNB takes the form of
1/m or (m − 1)/m, however, m adjacent tip vortices start
leapflogging and merging into a larger vortex, creating a
new vortex street with a lower spatial frequency. At other
frequency ratios, the tip vortices exhibit more irregular
deformation, suggesting a possible early breakdown into
turbulence. This has implications for both wind and tidal
farm design, where the interaction of downstream turbines
with the wakes of upstream turbines is an important
consideration.
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